Catalysis is an initiation or a change in the rate of a chemical reaction due to the participation of a material called a catalyst. Catalysts that speed the reaction are called positive catalysts. Catalysts that slow the reaction are called negative catalysts, or inhibitors. Unlike reactants, a catalyst is not consumed by the reaction itself.
A catalyst works by providing an alternative reaction pathway to the reaction product. The rate of the reaction is increased when this alternative route has a lower activation energy than the reaction route not mediated by the catalyst. Catalysts can also enable reactions that would otherwise be blocked or slowed by a kinetic barrier. The catalyst may increase reaction rate and/or selectivity, or enable the reaction to proceed at lower temperatures than would otherwise be possible. As such, catalysts can be very valuable tools in industrial processes. There can be drawbacks to the use of catalysts though. For example, tin compounds are used extensively in industrial products such as coatings, as catalysts for isocyanate/hydroxyl reactions. Unfortunately, often the catalyst levels required to provide acceptably fast cure rates and final product properties typically result in a short application time window after the components are mixed.
There is a need to work in a timely manner so that the mixed components maintain a low enough viscosity for spraying. The span of time during which the coating is ready to apply to a substrate and still of low enough viscosity to be applied is commonly referred to as “pot life.”
In the normal paradigm of catalyzed compositions, pot life and cure speed are typically in opposition to one another such that faster cure is facilitated by a higher catalyst concentration but at the expense of shorter pot life and vice versa. Longer pot life can be achieved with a lower catalyst concentration but will also lead to longer cure times. Typically, pot life must be balanced with cure speed of the applied coating. For instance, in a multi-component coating system that uses a catalyst, the pot life and cure speed are primarily controlled by the amount of catalyst present. Accordingly, if a fast cure speed is required more catalyst can be used but that will also cause a shorter pot life. Conversely, if a longer pot life is needed less catalyst can be used but the cure speed would also be retarded.
It is also important that the applied coating composition dry and harden quickly so that dirt pick-up is minimized and valuable shop space isn't occupied with the coated substrate, such as an automobile, while it is drying. The length of time between when a coating is applied to a substrate and when the coating has dried or cured sufficiently that dust falling onto the coated substrate will not stick to the coated substrate is referred to as “dust-free time” and is an indicator of the speed of cure. One way to speed the drying and cure of the composition is to add additional catalyst, but this shortens the time available for spraying since higher catalyst levels also cause viscosity of the composition to increase more quickly as reaction rates increase.
It would be desirable to catalyze chemical reactions using catalysts that overcome these drawbacks of the prior art by lengthening the pot life of the composition or by accelerating the reaction rate after application without adversely affecting the pot life.